The invention relates to a method for copy protecting a record carrier having information stored thereon according to predetermined formatting and error correcting rules, comprising the steps of creating an image file comprising main information, generating access control information for controlling the access to the main information, producing a master carrier in dependence on the image file and the access control information, which producing comprises the steps of creating a bit sequence by applying the formatting and error correcting rules to the image file and translating the bit sequence into a physical pattern of marks, and multiplicating the record carrier using the master carrier.
The invention further relates to a copy protected record carrier having a bit sequence stored thereon representing information according to predetermined formatting and error correcting rules, the information comprising main information and access control information for controlling the access to the main information.
The invention further relates to a method for detecting access control information on such a copy protected record carrier.
The invention further relates to a retrieval arrangement for retrieving information from such a copy protected record carrier, the arrangement comprising reading means for reading the record carrier, the reading means comprising a read unit for extracting a bit sequence stored on the record carrier and an error correcting unit for processing the bit sequence.
A system for copy protecting a record carrier, a copy protected record carrier and a reading arrangement are known from EP-0545472 (document D1 in the list of related documents). The known record carrier comprises a prearranged guiding track, a so-called pregroove. In the track determined by the pregroove, information which is written in a predefined manner is represented by optically readable patterns which are formed by variation of a first physical parameter, such as the height of the scanned surface. The pregroove has variations in a second physical parameter, such as an excursion in a transverse direction, also denoted as wobble. Pregroove wobble is FM-modulated and this modulation represents access control information which is related to information, such as a descramble code for recovering information stored as scrambled information. The known device comprises reading means for reading the patterns and recovering means for recovering the access control information. The known device and information carrier form a system for controlled information reproduction. For this purpose, the device comprises means for reproducing the information in dependence on the access control information. If the information is copied on a writable information carrier, the information of this copy will not be reproduced, because during the writing process only the patterns are written and the copy itself does not contain any access control information. A problem in the known system is that the reading means must be able to recover the access control information by detecting the variations of the second physical parameter.
It is an object of the invention to provide a system for copy protecting record carriers which does not rely on variations of physical parameters, while the making of usable copies on writable information carriers is counteracted.
For this purpose a method for copy protecting a record carrier as described in the opening paragraph, is characterized according to the invention in that in the producing step bits in the bit sequence are changed according to the access control information to constitute logical errors which cannot be corrected by said error correcting rules and which constitute an error pattern. A copy protected record carrier as described in the opening paragraph is characterized according to the invention in that the bit sequence comprises bit errors constituting logical errors which cannot be corrected by said error correcting rules and which constitute an error pattern representing at least part of the access control information. The measures are advantageous in that the error pattern can be easily detected, while errors cannot be included in information stored on a copy of the record carrier via a standard recording device, because such recording devices have built in error correcting rules, which cannot be manipulated.
According to a second aspect of the invention a method for detecting access control information on such a copy protected record carrier is characterized in that the method comprises the steps of selecting at least one error location, but not all error locations, which error location(s) should have a logical error according to the error pattern, and verifying the presence of an error by reading the selected error location(s). A retrieval arrangement for retrieving information from such a copy protected record carrier as described in the opening paragraph is characterized in that the arrangement comprises access control means for controlling the access to the information, which access control means are embodied so as to detect the access control information by selecting at least one error location, but not all error locations, which error location(s) should have a logical error according to the error pattern, and by verifying the presence of an error by reading the selected error location via the reading means. This is advantageous in that selecting a few error locations from an available larger number results in a fast response for the access control means. Reading a sector having errors might take up to 30 seconds on a standard reading device, such as a CD-ROM drive, due to automatic retries. Also selecting different locations for each access control session increases the difficulties for a malicious party in mimicking the access control process.
It is to be noted, that WO 95/03655 (document D3) describes a CD PROM encryption system, in which the information on a CD-ROM is encrypted by a key, which key is programmed into the CD-ROM after manufacture by damaging selected sectors to be unreadable by conventional read systems. The selected sectors are physically damaged by a high power laser. Record carriers are individually enabled by having a specific key for a certain user or group of users.
The invention is also based on the following recognition. Physically damaging selected parts of a record carrier leads to corrupted physical marks. When reading such damaged parts a read head may lose the track, or the translation from the read signal to a bit sequence may be disrupted or lose synchronisation, which all result in an uncontrollable number of errors. Moreover, physical damage may be easily detected from physical parameters and may be mimicked by a malicious party by physical means. Also, physically damaging sectors increases the production cost. The invention is based on a logical pattern of errors, which does not increase production cost and allows precise control of the resulting errors. In addition the inventors have had the insight, that physical errors cannot be used for generating errors in a limited part of the retrieved information, as they are similar to burst errors, which type of error is spread widely due to de-interleaving steps usually applied to the bit sequence in an error correcting and deformatting process for retrieving the information. Therefore, an embodiment of the copy protected record carrier is characterized in that, while the bit sequence comprises information bits and error correction bits, the information bits comprise the bit errors and/or in that the bit errors are located so as to accumulate in an error word uncorrectable by an error word correcting rule when reproducing. This is advantageous in that the bit errors are concentrated in error words resulting unambiguously in logical errors, while no error bits are spread to other parts of the retrieved information.
A further embodiment of the copy protected record carrier, which record carrier is subdivided into addressable sectors, is characterized in that the record carrier comprises a padding area, which padding area comprises error sectors and non-error sectors, which error sectors comprise the logical errors and constitute the error pattern. Using sectors as constituting elements for the error pattern has the advantage, that standard reading devices will read and process the read information sector by sector, while generating an error message if somewhere in a sector an uncorrectable error has been detected.
A further embodiment of the copy protected record carrier is characterized in that the part of the bit sequence corresponding to non-error sectors which adjoin error sectors comprises substantially no bit errors. In the case of some dirt or scratches the affected sectors will show random or small burst errors. If such errors had to be corrected in the presence of intentional bit errors, there would be an increased risk that a non-error sector would be classified as an error sector. Having substantially no intentional bit errors is advantageous in that there is a low probability of non-error sectors being wrongly classified.
A further embodiment of the copy protected record carrier is characterized in that, while the record carrier has a predetermined information storage capacity of which the main information covers a part, the padding area substantially covers the remaining part of the information storage capacity. This has the following advantage. All logical errors constituting the error pattern can only be detected by reading all of the padding area. On an average record carrier a relatively large percentage of the information storage capacity may be unused, which is available for the error pattern without the production cost increasing. At for example 20 seconds reading time for a sector having errors due to retries, it will take over 1000 hours to read the total padding area on a CD-ROM having 60% unused capacity.
An embodiment of the method for detecting access control information is characterized in that the method further comprises the steps of selecting at least one non-error location, but not all non-error locations, which non-error location(s) should not have a logical error according to the error pattern, and verifying the absence of an error by reading the selected non-error location(s). This is advantageous in that an illegal copy having also errors on non-error locations will be detected.
A further embodiment of the method for detecting access control information is characterized in that at least one non-error location is selected which adjoins an error location. This is advantageous in that an illegal copy having physical or burst type errors spreading over more locations due to de-interleaving rules will be detected.
Further advantageous, preferred embodiments of the copy protected record carrier, the retrieval arrangement and methods according to the invention are given in the further dependent claims.